I was wondering if turbulence can make the aircraft flip or just fall down..i must admit i get nervous when turbulence gets extreme and always wondering if the plane can actually flip or just fall down..a pilot who sat with me on a flight told me that's impossible and there has never been a crash due to turbulence..hmm any toughts?

Correct, that is impossible. The worst Turbulence can do is give you a bit of a bump around. The wings on these airplanes are built to withhold massive pressure and bend, so the possibility of the wing breaking during turbulence is out of the question.

And airplane can crash in a severe downdraft, which may feel like Turbulence at first, but infact is a rapid downburst of wind, usually on the tail of a severe thunderstorm and usually on final approach. CAT, the most common kind of Turbulence can cause no harm.

If you are flying a smaller airplane, like the ones I fly, then PERHAPS there is a greater possibility of some structure damage.

More appropriate for TravelBuzz! than AC. Please follow it there.

Sean
Moderator AC/Aeroplan

Someone like LarryJ can no doubt answer more accurately, but it might be helpful distinguish between structural failure and controllability, and b) (as noted) kinds of turbulence.

Aircraft are significantly over-engineered in terms of the amount of g-force that it would take to cause a structural failure of a critical component (like a wing coming off ) There are indeed instances of CAT turbulence causing g-forces that are large enough to damage an airframe and when such g-forces are encountered the aircraft has to have a mandatory major inspection. I am not recalling, though, any CAT incident with a large aircraft where CAT induced structural failure that caused a crash (but this being FT, I look forward to being corrected )

Crashes due to turbulence-related controllability issues are another matter. Low-altitude windshear on short final is one of the most deadly (witness the DL (?) crash many years ago at DFW). Windshear is defined as a sudden and significant change in direction or velocity of the wind relative to the aircraft's heading. When you are on short final, low and slow, and, say, the wind direction changes suddenly from a 20 knot headwind to a 20 knot tailwind, the aircraft is suddenly trying to fly at speed relative to the surrounding air that is slower than the aircraft's stall speed. The results can be unpleasant; if the aircraft is low enough to the ground there is not enough time for the aircraft engines to spool up to give enough thrust in time to counteract the loss of airspeed. This is why, since the DFW crash, a much better system of assessing and reporting windshear has been developed and why, when significant windshear is reported by other pilots or by the automated systems off the end of a runway an aircraft's pilot will carry extra airspeed.

A similar controllability issue exists for what's called wake turbulence. When an aircraft leaves the runway on takeoff/the wings begin producing lift very intense vortices (the intensity depending on the weight of the aircraft, to oversimplify) come off of the aircraft's wingtips. Any aircraft following, particularly one that is much lighter, can hit one of those invisible vortices as it falls toward the ground and can have a very rapid upset and get "spun" in the same direction as the vortices. This is why there are specific procedures for takeoff for pilots following such likely vortex inducing aircraft, including the requirement for greater time between a "heavy" aircraft taking off and a lighter one following it.

Mountain-wave/rotor turbulence is a third kind of turbulence that can make for controllability issues particularly in light aircraft. Very intense "spinning" vortices can develop over mountain ranges and no aircraft wants to get caught in one of those, particularly light aircraft.

And just a note on terminology: what you or I might consider "extreme" turbulence is often actually only mild or moderate turbulence per FAA definitions. (That doesn't mean it's enjoyable ) A professional pilot can go an entire career without encountering severe or extreme turbulence (these would mandate an extreme post-even inspection, too).

I like cblaisd's informative post. That just about covers it!

Don't worry about turbulence in terms of the safety of the aircraft itself. As cblaisd pointed out the aircraft are significantly over-engineered and can survive much worse turbulence then is ever experienced in-flight, especially since modern weather radars have made it much easier for pilots to avoid much of the bad weather in the first place. And when they do decide to fly around weather cells, it's primarily for the comfort of the passengers rather than safety.

The only real danger from turbulence is inside the airplane, from people and objects being tossed around the cabin. Even that is pretty rare, but that's why the airlines tell you to keep your seatbelt fastened even when seated. If you're buckled in, your chance of injury of any kind of turbulence is very low.

There is a youtube video of a 777 wing stress test that is amazing, and should restore your confidence that the wings will stay attached; the wing approaches near-vertical before breaking up.
IIRC the title is Boeing 777 wing ultimate load test. It's an incredible video.

Boeing 777 Ultimate Load Test

Indeed commercial aircraft are built very strongly and while turbulence at altitude may feel extreme, there is no way it's going to bring an aircraft down!

There were some very early jets (Comet, 707) that broke up due to direct turbulence stress, but I'm not aware of any recent ones.

As mentioned above, your biggest risk is being banged around the cabin, so wear your seat belt.

Thanks for the link. I wonder if the test results would have been different, however, with fluid in the wing fuel tanks?

The Alaska Air crash of the MD80 into the California Pacific might be informative of the stresses an aircraft can withstand.

Several eyewitness reports by overflying pilots related how the aircraft was pitched and even inverted in extreme attitudes, yet it remained structurally intact.

And that is with the aircraft payloaded and with the flight crew adding counter control inputs.

I was aboard a TG flight last month and we encountered turbulence a couple minutes after takeoff. Luckily people were still buckled in and there were no injuries. There was some stuff flying around the cabin and people were screaming like on a roller coaster .

The pilot later made an an announcement and apology. He said we experienced -1.3Gs and the plane was designed was designed for -2.5.

And undoubtedly, it could even withstand a significant amount more force than that, as there is always a safety margin beyond the design limits.

What I'd like to see is an explanation of the phenomenon of "mealtime turbulence:" as soon as the meal is on your tray and you take the foil off the little entree dish, the turbulence starts.

That's just one of the air travel variants of Murphy's Law.

Seriously, though, I don't notice this happening that often as I find that pilots are generally pretty good about keeping the FA's seating when they're expecting significant turbulence.